220 Journal of Agricultural Research voi. xiii, N0.4 



With clay loam the results are quite uniform until 0.300 per cent of 

 salt is added, when there occurs a sharp decrease in the amount of 

 ammonia produced. This is the only salt tested which gives results of 

 this kind with clay loam. With the other salts the decrease in ammoni- 

 fication is gradual, no special point being observed where a marked de- 

 crease in ammonification occurs. As in the results -with sandy loam, an 

 increase in the osmotic pressure of the soil solution is observed some- 

 what before any decrease in bacterial activities are evident. 



GENERAL DISCUSSION 



The data presented bring out, first of all, that there is a decided 

 difference in the effect on ammonification of the salts when applied to 

 different soils. With every salt tested there was found with sandy 

 loam a point at which marked depression of the ammonifying power 

 occurred, while with clay loam such a point was found for potassium 

 chlorid only. Whether this difference is due to a difference in the flora 

 present or to the physical or chemical properties of the soil is unde- 

 termined. In the light of the work of Bouyoucos and McCool ' and 

 McCool and Wheeting,^ however, there is unquestionably a difference in 

 the composition of the soil solution. The difference in the behavior of 

 these soils emphasizes the importance of much thorough investigation 

 of the problem of toxicity of salts in soils. 



Another point of interest is the lack of regularity in the results after a 

 point is reached where ammonification is markedly depressed. It might 

 be expected, whether the depression is due to toxicity or osmotic pressure, 

 that from this point every addition of salt would cause an ever-increasing 

 amount of depression. In no case, however, is this found to be true. 

 Where there is a consistent decrease in ammonia production after this 

 critical point is passed, such as occurs with calcium nitrate, the decreases 

 are small. In many cases there are fluctuations, such as occur with potas- 

 sium chlorid and magnesium sulphate, which are generally ascribed to 

 experimental error. Again there are cases where further additions of 

 salt fail to exert any further depressing effect on the ammonifying 

 organisms, even though the amount of salt added may be increased 

 four times, as is seen in the tests with calcium chlorid. It seems when 

 such results as referred to above are obtained that there must be some 

 fundamental cause for the variation, a possible explanation being com- 

 plex chemical interchanges between the salts and the soil constituents, 

 the end products depending on the amount of salt present. That some 

 such factor is operative is made more evident by such results as those 



' Bouyoucos, G.J,, and McCool, M. M. Op. cit. 

 'McCooi., M. M., AKD Whbeting, h. C. Op. cit. 



